1. Field of the Invention:
The present invention relates generally to a method and apparatus for recording a digital information signal and, more specifically, relates to recording a digital information signal and a pilot signal using a rotary head and employing the pilot signal to control the tracking of the head during playback.
2. Description of the Art:
It is well known to pulse-code-modulate (PCM) a video signal and/or an audio signal for recording on a magnetic tape as a series of parallel, slanted tracks using a rotary head assembly of the helical scan kind. Subsequently, during reproduction of the recorded signals, the PCM signals are then appropriately demodulated back to the original analog form. The principal reason for such pulse-code-modulation of the video signal and the audio signal is that in that form the signals can be recorded and reproduced with much higher quality than the original analog signals.
In a video tape recorder the conventional tracking control system enables the rotary head to correctly trace the appropriate track upon playback and employs a control signal recorded along the length of the tape. This control signal is recorded and reproduced using a fixed stationary head, not the rotary head. Using the reproduced control signal from the longitudinal track, the rotational phase of the rotary head is maintained in a constant phase relationship. The fixed magnetic head that is required for this kind of tracking control, however, tends to prevent the recording and reproducing apparatus from being of compact size, because the fixed head requires its own mounting space and associated mounting hardware, all of which must be located inside the compact apparatus and arranged adjacent the tape guide drum.
Therefore, various approaches have been proposed to provide tracking control without requiring a separate fixed head. In one such previously proposed approach, an analog video signal is recorded and/or reproduced in a so-called superimposed condition, with no guard bands being formed between the adjacent tracks. In the superimposed condition, each rotary head has a different azimuth angle and each head is slightly wider than the resultant recorded track. This is accomplished during recording by having the edge portion of each successive track laid down by the respective head overlap the adjacent edge portion of the next preceding track. Each track then both overlaps the edge of the preceding track and has its opposite edge overlapped as well by the next successive track, thereby leading to the description, "superimposed." the edge portions of the two adjacent tracks that will be traced by the wide head during playback will not present a reproduction problem because these adjacent tracks were recorded by a head having a different azimuth gap angle than the track being reproduced and those signals will not be reproduced with a level that could degrade the desired signal. In this superimposed tracking control approach, four different kinds (frequencies) of tracking pilot signals are recorded by the rotary heads on the tracks in which the video signal is recorded in the superimposed state. The pilot signals are selected to have a low frequency relative to the video signal being recorded, and are in a region outside the frequency spectrum of the video signals being recorded, so that upon playback they can be easily separated one from another.
Nevertheless, a principal feature of this tracking control method becomes its major drawback because signals having relatively low frequencies, such as these low-frequency pilot signals, are difficult to erase. Accordingly, when the pilot signal is erased in making a new recording following this superimposed approach, a portion of the previously recorded pilot signal remains unerased. Furthermore, depending upon the modulation system employed, when the information signal to be recorded has a frequency spectrum in the low-frequency band region, such as the PCM signal would have, it becomes extremely difficult to separate the pilot signals from the signals of interest. Moreover, since this tracking control method employs four kinds of pilot signals, that is, signals having four different frequencies, the attendant circuitry becomes complex and expensive. Another approach to providing tracking control in a rotary head video tape recorder is taught in U.S. Pat. No. 4,141,048, assigned to the assignee hereof, in which the pilot signal is recorded in the horizontal blanking period of the video signal. Nevertheless, since the pilot signal is in the same frequency band region as the video signal, it is once again difficult to reproduce the pilot signal accurately when reproducing the recorded video signal.